1. Field of the Invention
The present invention relates to a method for seeking and setting a resonance frequency of the type wherein a predetermined frequency interval is swept to locate a resonant frequency in the intervals, as well as a tuner operating according to the method.
2. Description of the Prior Art
Ultrasound is often used e.g. for nebulizing (atomizing) medication to be delivered to a patient connected to a ventilator. Ultrasound can be appropriately generated with a piezoelectric crystal. The crystal is then a frequency-determining component in an oscillator. The frequency of the ultrasound then depends on the resonant frequency. The acousto-mechanical model is complex for this type of oscillator, especially when electrical impedances in the signal transmission etc. are taken into account. There is then the risk that more than one resonant peak could lie with a specific frequency range. Only one of these peaks coincides with mechanical resonance and elicits the largest output from the crystal. The piezoelectric crystal can be viewed as a load impedance.
In principle, "load impedance" in this description refers to all kinds of loads, i.e. a load which can be wholly resistive, wholly inductive, wholly capacitive or a mixture of two or more of these. The character of the load also can change during operation and, e.g., vary from inductive to capacitive.
One way to find the resonant frequency is to sweep across a predetermined frequency interval and then lock modulation to the frequency yielding the highest peak.
As a result of signal drift, temperature drift etc., problems will develop when the modulation fails to produce the right frequency over time.
One way to solve this problem has been to use a phase locked loop (PLL). This means that an error signal, corresponding to drift in the system, is determined, and modulation is reset to the preset frequency.
Neither of these methods takes into account the fact that the mechanical resonance frequency can change for some reason. One such change can be caused by changes in the mechanical load on the crystal or changes in temperature.
Another problem is also that there are usually several variables involved during the searching for a resonant frequency, all of which influence the searching. Overshooting also is a problem that mainly occurs when a sweep is made at a relatively high speed in relation to time constants of components etc. An overshoot may lead to less accuracy in identifying the proper resonant frequency.